Microbial toxin proteins are used in medicine, as immunogens for vaccination against the toxin-producing microbe and as carrier proteins and adjuvants for other vaccines, and in scientific research as tools for studying molecular pathways.
Diphtheria toxin (DT) is a proteinaceous toxin that is synthesized and secreted by toxigenic strains of Corynebacterium diphtheriae. Toxigenic strains contain a bacteriophage lysogen carrying the toxin gene. DT is synthesized as a 535-amino-acid polypeptide, which undergoes proteolysis to form the mature toxin. The mature toxin comprises two subunits, A and B, joined by a disulfide bridge. The B subunit, formed from the C-terminal portion of intact DT, enables binding and entry of DT through the cell membrane and into the cytoplasm. Upon cell entry, the enzymatic A subunit, formed from the N terminal portion of intact DT, catalyzes ADP ribosylation of Elongation Factor 2 (EF-2). As a result, EF-2 is inactivated, protein synthesis stops, and the cell dies. Diphtheria toxin is highly cytotoxic; a single molecule can be lethal to a cell, and a dose of 10 ng/kg can kill animals and humans.
The CRM197 protein is a nontoxic, immunologically cross-reacting form of DT. It has been studied for its potential use as a DT booster or vaccine antigen. CRM197 is produced by C. diphtheriae that has been infected by the nontoxigenic phage β197tox-created by nitrosoguanidine mutagenesis of the toxigenic corynephage β. The CRM197 protein has the same molecular weight as DT but differs by a single base change (guanine to adenine) in the A subunit. This single base change results in an amino acid substitution (glutamic acid for glycine) and eliminates the toxic properties of DT.
Conjugated polysaccharide vaccines that use CRM197 as a carrier protein have been approved for human use. Vaccines include: Menveo® (Novartis Vaccines and Diagnostics), a vaccine indicated for preventing invasive meningococcal disease caused by Neisseria meningitidis subgroups A, C, Y, and W-135; Menjugate (Novartis Vaccines), a meningococcal group C conjugate vaccine; and Prevnar® (Wyeth Pharmaceuticals, Inc.), a childhood pneumonia vaccine that targets seven serotypes of Streptococcus pneumoniae, and HibTITER® (Wyeth), a Haemophilus influenzae type b vaccine. In addition, CRM197 has potential use as a boosting antigen for C. diphtheria vaccination and is being investigated as a carrier protein for use in other vaccines.
A method for high-level expression of CRM197 for approved therapeutics and investigational use has not been reported. CRM197 has been expressed in, e.g., C. diphtheriae, B. subtilis, and E. coli, at levels that range in the tens of mg/L. A single dose of the Prevnar conjugate vaccine contains about 20 μg of CRM197. Therefore, a method for economically producing CRM197 at levels of about 1 g/L or more would greatly facilitate vaccine research and manufacture.
Cholera Toxin (CTX), produced by Vibrio cholera, a bacterial pathogen that causes an infection characterized by diarrhea and vomiting, is also an ADP-ribosylating toxin. CTX is an oligomeric complex made up of six protein subunits: a single copy of the Cholera toxin A subunit (CTA), and five copies of the Cholera Toxin B subunit (CTB). The five B subunits, each weighing 12 kDa, form a five-membered ring. The A subunit has an A1 portion, CTA1, a globular enzyme that ADP-ribosylates G proteins, and an A2 chain, CTA2, that forms an extended alpha helix which sits snugly in the central pore of the B subunit ring. This ring binds to GM1 ganglioside receptors on the host cell surface, resulting in internalization of the entire complex. Once internalized, the CTA1 chain is released by reduction of a disulfide bridge. CTA1 is then activated and catalyzes ADP ribosylation of adenylate cyclase. The resulting increase in adenylate cyclase activity increases cyclic AMP synthesis, which causes massive fluid and electrolyte efflux and results in diarrhea.
The B subunit of CTX, though relatively harmless, retains its ability to bind to the GM1 ganglioside receptor. CTB therefore finds use in facilitating mucosal uptake of chemically or genetically conjugated foreign antigens. It has been demonstrated to induce both mucosal and systemic immunity, and is a candidate for use in edible vaccine production. Because of its binding preference, CTB also finds use as a neuronal tracer.
Pertussis toxin (PTX) is an exotoxin and virulence factor produced by Bordetella pertussis, a bacterial pathogen of the human respiratory tract that causes the disease whooping cough. The pertussis holotoxin is a multi-subunit complex with an AB 5 structure. The enzymatically active A subunit (S1) is an ADP-ribosyltransferase that modifies the alpha subunit of several heterotrimeric G proteins in mammalian cells, and the B oligomer (S2, S3, two copies of S4, and S5) binds glycoconjugate receptors on cells. The five subunits of the toxin are expressed from the Pertussis Toxoid operon.
Nontoxic variants of Pertussis toxin have been explored for use in protective vaccines and as a vaccine adjuvant. There is also a need for Pertussis toxin protein to use in research, e.g., for studies of G protein signaling pathways.
Tetanus Toxin, produced by Clostridium tetani, is a neurotoxin having a molecular weight of 150 kDa. It is made up of two parts: a 100 kDa heavy or B-chain and a 50 kDa light or A-chain. The chains are connected by a disulfide bond. The B-chain binds to disialogangliosides (GD2 and GD1b) on the neuronal membrane. The A-chain, a zinc endopeptidase, attacks the vesicle-associated membrane protein (VAMP).
The action of the A-chain stops the affected neurons from releasing the inhibitory neurotransmitters GABA (gamma-aminobutyric acid) and glycine by degrading the protein synaptobrevin. The consequence of this is dangerous overactivity in the muscles from the smallest stimulus—the failure of inhibition of motor reflexes by sensory stimulation. This causes generalized contractions of the agonist and antagonist musculature, termed a tetanic spasm.
Tetanus Toxin Fragment C (Tet C or TTC) is a 50 kD polypeptide generated by protease cleavage (e.g., with papain) of Tetanus toxin, or through recombinant expression of the fragment. It corresponds to the 451 amino acids at the C-terminus (amino acid positions 865-1315).
Fragment C has been shown to be non-toxic. Because it binds to neurons with high specificity and affinity, TTC finds use as a targeting molecule for neuronal drug delivery or for research purposes. TTC protein is also potentially useful as a vaccine carrier protein and for use in a vaccine to protect against C. tetani infection.
Clostridium difficile Toxin B (TcdB) is a virulence factor produced by Clostridium difficile, which causes hospital acquired diarrhea and pseudomembranous colitis. TcdB, and a second large clostridial toxin, TcdA, are involved in the development of pseudomembranous colitis.
TcdB is a glucosylating toxin of about 270 kD, and can be divided into enzymatic, translocation and receptor binding domains. The first 546 amino acids of TcdB contain the enzymatic region, which is followed by a putative translocation and receptor-binding domain. TcdB has potential use as a protective vaccine for C. difficile infection, as well as in diagnostic tests and their development.
Exotoxin A (ETA or PE) of Pseudomonas aeruginosa is a Type II ADPRT. Like its family members Diphtheria toxin and Cholera Toxin, it inhibits protein synthesis by the ADP-ribosylation of cellular elongation factor 2. P. aeruginosa Exotoxin A exists as a monomer, consisting of a single polypeptide chain of 613 amino acids (66 Kd).
ETA is potentially useful as a vaccine conjugate. Nontoxic mutants of ETA have been studied as vaccine conjugates for vaccinations that protect against Staphylococcus aureus, malaria, and Salmonella Typhi. 
Producing these toxins in amounts sufficient to meet expanding needs has presented significant challenges. When made in conventional protein overexpression systems, the toxin proteins are recovered in active form only at very low concentration due to degradation, improper folding, or both, depending on the specific characteristics, e.g., size and secondary structure, of the toxin. Therefore, methods for producing large amounts of these toxins, in soluble and/or active form, and at low cost is needed.